Fuel delivery system having a fuel oxygen reduction unit

ABSTRACT

A fuel system for an aircraft includes a fuel source and a fuel oxygen reduction unit defining a liquid fuel supply path, a stripping gas supply path, a liquid fuel outlet path, and a stripping gas return path, wherein the stripping gas return path and stripping gas supply path are each in airflow communication with the fuel source.

FIELD

The present subject matter relates generally to a fuel oxygen reduction unit for a fuel delivery system of a vehicle.

BACKGROUND

Typical aircraft propulsion systems include one or more gas turbine engines. The gas turbine engines generally include a turbomachine, the turbomachine including, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. In operation, air is provided to an inlet of the compressor section where one or more axial compressors progressively compress the air until it reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section to the turbine section. The flow of combustion gasses through the turbine section drives the turbine section and is then routed through the exhaust section, e.g., to atmosphere.

Certain operations and systems of the gas turbine engines and aircraft may generate a relatively large amount of heat. Fuel has been determined to be an efficient heat sink to receive at least some of such heat during operations due at least in part to its heat capacity and an increased efficiency in combustion operations that may result from combusting higher temperature fuel. However, heating the fuel up without properly conditioning the fuel may cause the fuel to “coke,” or form solid particles that may clog up certain components of the fuel system, such as the fuel nozzles. Reducing an amount of oxygen in the fuel may effectively reduce the likelihood that the fuel will coke beyond an unacceptable amount. Fuel oxygen reduction systems have been proposed for such a purpose. These fuel oxygen reduction systems are generally positioned at the engine to reduce an oxygen content of the fuel provided to the engine.

In addition, certain fuel delivery systems for aircraft including gas turbine engines generally also include a system for removing an oxygen content of an air within an ullage of a fuel tank (e.g., the fuel tank from which fuel is provided to the gas turbine engines). Reducing an oxygen content of such gasses may reduce a risk of fire within the fuel tank. The system for removing an oxygen content of the air/gas in the ullage generally operate differently than the fuel oxygen reduction system (for reducing an oxygen content of a liquid fuel).

The inventors of the present disclosure have found that operating each of these various systems may be inefficient. Accordingly, a system for reducing both an oxygen content of a liquid fuel provided to each individual aircraft engine and of gasses within an ullage of the fuel tank would be useful.

BRIEF DESCRIPTION

Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

In one exemplary embodiment of the present disclosure, a fuel system for an aircraft is provided. The system includes a fuel source and a fuel oxygen reduction unit defining a liquid fuel supply path, a stripping gas supply path, a liquid fuel outlet path, and a stripping gas return path, wherein the stripping gas return path and stripping gas supply path are each in airflow communication with the fuel source.

In certain exemplary embodiments the fuel source further includes a fuel tank, and wherein the stripping gas supply path and the stripping gas return path are each in airflow communication with an ullage of the fuel tank.

For example, in certain exemplary embodiments the liquid fuel supply path is fluidly connected to the fuel source, and wherein the fuel oxygen reduction unit is configured to receive a flow of liquid fuel from the fuel source.

In certain exemplary embodiments the fuel oxygen reduction unit is configured to receive a flow of gas through the stripping gas supply path and a flow of liquid fuel through the liquid fuel supply path, and wherein the fuel oxygen reduction unit is configured to reduce an oxygen content of the flow of gas and the flow of liquid fuel.

In certain exemplary embodiments the fuel oxygen reduction unit further includes a gas oxygen reduction unit positioned in the stripping gas supply path or the stripping gas return path; and a fuel oxygen reduction assembly in fluid communication with the liquid fuel supply path and the liquid fuel return path, and further in airflow communication with the stripping gas supply path and stripping gas return path.

For example, in certain exemplary embodiments the gas oxygen reduction unit is a first gas oxygen reduction unit positioned in the stripping gas supply path, and wherein the fuel oxygen reduction unit further includes a second gas oxygen reduction unit in the stripping gas return path.

For example, in certain exemplary embodiments the fuel oxygen reduction assembly further includes a contactor fluidly connected to the liquid fuel supply path and the stripping gas supply path for mixing a liquid fuel flow from the liquid fuel supply path with a stripping gas flow from the stripping gas supply path to form a fuel/gas mixture; and a separator fluidly connected to the contactor for receiving the fuel/gas mixture and separating the fuel/gas mixture back into the stripping gas flow and the liquid fuel flow, the separator fluidly connected to the liquid fuel outlet path and the stripping gas return path for providing the liquid fuel flow to the liquid fuel outlet path and the stripping gas flow to the stripping gas return path.

For example, in certain exemplary embodiments the fuel oxygen reduction assembly further includes a membrane fuel oxygen reduction unit defining a stripping gas chamber in airflow communication with the stripping gas supply path and the stripping gas return path, and a fuel chamber in fluid communication with liquid fuel supply path and the liquid fuel outlet path, the membrane fuel oxygen reduction unit including a membrane separating the stripping gas chamber from the fuel chamber.

For example, in certain exemplary embodiments the gas oxygen reduction unit is a membrane gas oxygen reduction unit.

For example, in certain exemplary embodiments the gas oxygen reduction unit is a catalyst.

For example, in certain other exemplary embodiments the fuel oxygen reduction unit further includes a pre-heater thermally coupled to the stripping gas supply path; and a heat exchanger thermally coupled to the stripping gas return path.

In certain exemplary embodiments the fuel oxygen reduction unit further includes an ullage pump in airflow communication with the stripping gas supply path.

In another exemplary embodiment of the present disclosure, a propulsion system for an aircraft is provided. The system includes an aircraft engine; and a fuel system. The fuel system includes a fuel source; and a fuel oxygen reduction unit defining a liquid fuel supply path, a stripping gas supply path, a liquid fuel outlet path, and a stripping gas return path, wherein the stripping gas return path and stripping gas supply path are each in airflow communication with the fuel source, and wherein the liquid fuel outlet path is in fluid communication with the aircraft engine for providing liquid fuel to the aircraft engine.

In certain exemplary embodiments the aircraft engine is a first aircraft engine, and wherein the propulsion system further includes a second aircraft engine, wherein the liquid fuel outlet path is in fluid communication with both the first aircraft engine and the second aircraft engine for providing liquid fuel to both the first aircraft engine and the second aircraft engine.

In certain exemplary embodiments the fuel oxygen reduction unit further includes a gas oxygen reduction unit positioned in the stripping gas supply path or the stripping gas return path; and a fuel oxygen reduction assembly in fluid communication with the liquid fuel supply path and the liquid fuel return path, and further in airflow communication with the stripping gas supply path and stripping gas return path.

For example, in certain exemplary embodiments the gas oxygen reduction unit is a first gas oxygen reduction unit positioned in the stripping gas supply path, and wherein the fuel oxygen reduction unit further includes a second gas oxygen reduction unit in the stripping gas return path.

For example, in certain exemplary embodiments the fuel oxygen reduction assembly further includes a contactor fluidly connected to the liquid fuel supply path and the stripping gas supply path for mixing a liquid fuel flow from the liquid fuel supply path with a stripping gas flow from the stripping gas supply path to form a fuel/gas mixture; and a separator fluidly connected to the contactor for receiving the fuel/gas mixture and separating the fuel/gas mixture back into the stripping gas flow and the liquid fuel flow, the separator fluidly connected to the liquid fuel outlet path and the stripping gas return path for providing the liquid fuel flow to the liquid fuel outlet path and the stripping gas flow to the stripping gas return path.

For example, in certain exemplary embodiments the fuel oxygen reduction assembly further includes a membrane fuel oxygen reduction unit defining a stripping gas chamber in airflow communication with the stripping gas supply path and the stripping gas return path, and a fuel chamber in fluid communication with liquid fuel supply path and the liquid fuel outlet path, the membrane fuel oxygen reduction unit including a membrane separating the stripping gas chamber from the fuel chamber.

For example, in certain exemplary embodiments the gas oxygen reduction unit is a membrane gas oxygen reduction unit or a catalyst.

In certain exemplary embodiments the liquid fuel supply path is fluidly connected to the fuel source for receiving a liquid fuel flow from the fuel source.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

FIG. 1 is a top view of an aircraft according to various exemplary embodiments of the present disclosure.

FIG. 2 is a schematic view of a fuel delivery system in accordance with an exemplary embodiment of the present disclosure.

FIG. 3 is a close-up, schematic view of a section of a fuel delivery system in accordance with an exemplary embodiment of the present disclosure.

FIG. 4 is a schematic, close-up, cross-sectional view of a fuel gas separator of an exemplary fuel oxygen reduction unit of the fuel delivery system of FIG. 3 in accordance with an exemplary embodiment of the present disclosure.

FIG. 5 is a close-up, schematic view of a section of a fuel delivery system in accordance with an exemplary embodiment of the present disclosure.

FIG. 6 is a close-up, schematic view of a section of a fuel delivery system in accordance with another exemplary embodiment of the present disclosure.

FIG. 7 is a close-up, schematic view of a section of a fuel delivery system in accordance with yet another exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention.

As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components.

The terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows.

The terms “coupled,” “fixed,” “attached to,” and the like refer to both direct coupling, fixing, or attaching, as well as indirect coupling, fixing, or attaching through one or more intermediate components or features, unless otherwise specified herein.

The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise.

Approximating language, as used herein throughout the specification and claims, is applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. For example, the approximating language may refer to being within a 10 percent margin.

Here and throughout the specification and claims, range limitations are combined and interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. For example, all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other.

Referring now to the drawings, wherein identical numerals indicate the same elements throughout the figures, FIG. 1 provides a top view of an exemplary aircraft 10 as may incorporate various embodiments of the present invention. The aircraft 10 defines a longitudinal centerline 14 that extends therethrough, a vertical direction (not shown), a lateral direction L, a forward end 16, and an aft end 18. Moreover, the aircraft 10 includes a fuselage 12, extending longitudinally from the forward end 16 of the aircraft 10 towards the aft end 18 of the aircraft 10, and a pair of wings 20. A first wing 20A of such wings 20 extends laterally outwardly with respect to the longitudinal centerline 14 from a port side 22 of the fuselage 12 and a second wing 20B of such wings 20 extends laterally outwardly with respect to the longitudinal centerline 14 from a starboard side 24 of the fuselage 12. Each of the wings 20 for the exemplary embodiment depicted includes one or more leading edge flaps 26 and one or more trailing edge flaps 28. The aircraft 10 further includes a vertical stabilizer 30 having a rudder flap 32 for yaw control, and a pair of horizontal stabilizers 34, each having an elevator flap 36 for pitch control. The fuselage 12 additionally includes an outer surface or skin 38. It should be appreciated however, that in other exemplary embodiments of the present disclosure, the aircraft 10 may additionally or alternatively include any other suitable configuration of stabilizer that may or may not extend directly along the vertical direction or horizontal/lateral direction L. Further, the aircraft 10 may include any other suitable configuration of wings 20 (e.g., a canard configuration, blended wing configuration, etc.), fuselage 12, etc.

The exemplary aircraft 10 of FIG. 1 includes a propulsion system 50, herein referred to as “system 50”. The exemplary system 50 includes one or more aircraft engines 52. For example, the embodiment depicted includes a plurality of aircraft engines 52, each configured to be mounted to the aircraft 10, such as to one of the pair of wings 20. More specifically, for the embodiment depicted, the aircraft engines 52 are configured as gas turbine engines, such as turbofan jet engines, attached to and suspended beneath the wings 20 in an under-wing configuration (it being appreciated that in other embodiments, the aircraft engines 52 may be any other suitable, at least partially combustion engines). For example, the aircraft engines 52 include a first aircraft engine 52A and a second aircraft engine 52B. The first aircraft engine 52A is configured to be mounted to the first wing 20A of the aircraft 10, and the second aircraft engine 52 is configured to be mounted to the second wing 20B of the aircraft 10 (i.e., on an opposite side of the aircraft 10). It will be appreciated that although only two aircraft engines 52 are depicted in FIG. 1, in other embodiments, the propulsion system 50 of the aircraft 10 may have any other suitable number and type of aircraft engines 52, mounted at any other suitable location (e.g., mounted to the wings, to the fuselage at the aft end of the aircraft 10, to one or stabilizers, etc.).

As is further depicted in FIG. 1, the aircraft 10 includes a fuel delivery system 100. More specifically, the fuel delivery system 100 is generally configured to supply the aircraft engines 52, i.e., the first aircraft engine 52A and the second aircraft engine 52B for the embodiment of FIG. 1, with an amount of fuel during operation. More specifically, for the embodiment shown, the fuel delivery system 100 is configured to provide the aircraft engines 52 with a relatively low oxygen content fuel during operation, as will be explained in greater detail below. In such a manner, it will be appreciated that the fuel delivery system 100 generally includes a fuel source 102 (e.g., a fuel tank) and a fuel oxygen reduction unit 104. The fuel oxygen reduction unit 104 is configured to receive fuel from fuel source 102, reduce an oxygen content of such fuel, and provide such fuel to the aircraft engines 52. Moreover, as will be explained in more detail with reference to FIGS. 2 and 3, below, the fuel oxygen reduction system 104 is further configured to reduce an oxygen content of an air within an ullage of the fuel source 102. Notably, as used herein, the term “fuel oxygen reduction unit” generally means a device capable of reducing a free oxygen content of the fuel, such as a fuel oxygen conversion unit, a fuel oxygen extraction unit, etc.

It will be appreciated, however, that in other exemplary embodiments, the aircraft 10 and/or engines 52 may have any other suitable configuration. For example, in other embodiments, the aircraft 10 may have other wing and/or fuselage designs, engine count and/or configuration or positioning, etc. Further, in other embodiments, the aircraft 10 may be, e.g., a vertical takeoff and landing aircraft, such as a helicopter. Other embodiments are contemplated as well.

More specifically, referring now briefly to FIG. 2, a schematic view of a fuel delivery system 100 as may be incorporated into the aircraft 10 of FIG. 1 is provided. As is depicted, the fuel delivery system 100 generally includes a fuel source 102 and a fuel oxygen reduction unit 104, and defines a plurality of flowpaths. Specifically, for the embodiment shown, the fuel delivery system 100, and the fuel oxygen reduction unit 104, generally defines a liquid fuel supply path 106, a stripping gas supply path 108, a liquid fuel outlet path 110, and a stripping gas return path 112. The stripping gas supply path 108 and stripping gas return path 112 are each in airflow communication with the fuel source 102 for receiving a flow of gas 114 from the fuel source 102 (which, as will be explained below, is a stripping gas) and providing the flow of gas 114 back to the fuel source 102. Similarly, for the embodiment shown, the liquid fuel supply path 106 is in fluid communication with the fuel source 102 for receiving a flow of liquid fuel 116 therefrom. Notably, for the embodiment shown, the fuel source 102 is configured as a fuel tank. Within the fuel tank, the fuel source 102 includes an amount of liquid fuel and an amount of vapor/air/gas. The space within the tank containing the vapor/air/gas (e.g., within the tank and above the fuel) may generally be referred to as the ullage 118. During operation, it will be appreciated that the fuel oxygen reduction unit 104 is generally configured to: receive the flow of air/gas 114 from the ullage 118 of the fuel source 102 and the flow of liquid fuel 116 from the fuel source 102, to reduce an oxygen content of each, to provide the relatively low oxygen content flow of gas 114 back to the ullage 118, and to provide the relatively low oxygen content flow of liquid fuel 116 to the aircraft engines 52. By reducing an oxygen content of the vapor/air/gas in the ullage 118, a risk of flame-ups or fires within the tank in case of a spark may be reduced. Further, by reducing an oxygen content of the liquid fuel 116 provided to the engines 52, an increased amount of heat may be added thereto with reduced risk of coking (allowing for increased efficiencies in the combustion process and providing an effective heat sink).

Specifically, for the embodiment shown, the fuel delivery system 100 is configured to provide fuel 116 having a relatively low oxygen content from the fuel oxygen reduction unit 104 to each of the plurality of aircraft engines 52, which for the embodiment shown includes a first aircraft engine 52A, a second aircraft engine 52B, up to an “Nth” aircraft engine 52N (e.g., three, four, five, six, etc. aircraft engines 52). In such a manner, it will be appreciated that the fuel deoxygenation unit 104 is fluidly coupled to each of the plurality of aircraft engines 52 through the liquid fuel outlet path 110, and is therefore configured to provide relatively low oxygen content liquid fuel 116 to each of such engines 52.

A fuel delivery system 100 in accordance with such an exemplary embodiment, as will be discussed in greater detail below, may be capable of maintaining a relatively low oxygen content air within the ullage 118, while also providing relatively low oxygen content fuel to a plurality of aircraft engines 52.

Notably, for the embodiment shown fuel delivery system 100 further includes a return valve 120 and a return line 122 for returning an amount of relatively low oxygen content liquid fuel to the fuel source 102 in the event such fuel is not required by the aircraft engines 52. However, in other embodiments, the system 100 may be configured without the return valve 120 and return line 122 and may instead regulate the flow of liquid fuel 116 in other suitable manners.

Referring now to FIG. 3, a more detailed, schematic drawing of a fuel delivery system 100 for an aircraft 10 in accordance with an exemplary embodiment of the present disclosure is provided. In at least certain exemplary embodiments, the exemplary fuel delivery system 100 depicted in FIG. 3 may be incorporated into, e.g., the exemplary aircraft 10 described above with reference to FIG. 1, and/or may be configured in a similar manner to the exemplary fuel delivery system 100 of FIG. 2. As such, it will be appreciated that the fuel delivery system 100 generally includes a fuel source 102 and a fuel oxygen reduction unit 104, and the fuel delivery system 100 and fuel oxygen reduction unit 104 generally defines a liquid fuel supply path 106, a stripping gas supply path 108, a liquid fuel outlet path 110, and a stripping gas return path 112. The stripping gas return path 112 and the stripping gas supply path 108 are each in airflow communication with the fuel source 102. Further, for the embodiment depicted, the liquid fuel supply path 106 of the fuel oxygen reduction unit 104 is in fluid communication with fuel source 102. In such a manner, it will be appreciated that the fuel oxygen reduction unit 104 is generally configured to receive a flow of liquid fuel 116 from the fuel source 102 and is further configured to receive a flow of gas 114 (referred to herein as “stripping gas”) also from the fuel source 102. Further, as will be described in detail below, the fuel oxygen reduction unit 104 is configured to return the stripping gas 114 to the fuel source 102. As noted with reference to the embodiment above, for the embodiment of FIG. 3, the fuel source 102 generally includes an amount of liquid fuel 116 and defines an ullage 118 above the liquid fuel 116. The gas received through the stripping gas supply path 108 is received from the ullage 118, and the gas provided to the fuel source 102 through the stripping gas return path 112 is provided to the ullage 118.

In order to assist with the flow of stripping gas 114 from the fuel source 102, or rather from the ullage 118 of the fuel source 102, the fuel delivery system 100 includes an ullage pump 124 in airflow communication with the ullage 118 of the fuel source 102 and the stripping gas supply path 108 of the fuel oxygen reduction unit 104. Similarly, in order to assist with the flow of liquid fuel 116 from the fuel source 102, the fuel delivery system 100 includes a fuel pump 126 in fluid communication with the fuel source 102 and the liquid fuel supply path 106.

Further, for the embodiment depicted, the fuel oxygen reduction unit 104 generally includes a contactor 128 and a fuel gas separator 130. Additionally, the exemplary fuel oxygen reduction unit 104 defines a stripping gas flowpath 132. For the embodiment shown, the stripping gas flowpath 132 includes the stripping gas supply path 108 and the stripping gas return path 112. As such, the stripping gas flowpath 132 generally extends from a location upstream of the contactor 128, through the contactor 128 and separator 130, to a location downstream of the separator 130. In certain exemplary embodiments, the stripping gas flowpath 132 may be formed of any combination of one or more conduits, tubes, pipes, etc., as well as structures of components within the stripping gas flowpath 132.

It will be appreciated that the term “stripping gas” is used herein as a term of convenience to refer to a gas generally capable of performing the functions described herein. Specifically, for the embodiment depicted, the stripping gas 114 is substantially comprised of gas from the ullage 118 of the fuel source 102. As such, the stripping gas 114 may be made up primarily of atmospheric air. Additionally, or alternatively, the stripping gas 114 may be any other suitable gas, such as an inert gas or a substantially inert gas.

Referring still to the embodiment depicted, the fuel gas separator 130 is a mechanically-driven fuel gas separator 130 mechanically coupled to, and driven by, a power source 134. For the embodiment of FIG. 3, the power source 134 driving the fuel gas separator 130 may be any suitable power source, such as an electric motor, a hydraulic motor, a pneumatic motor, a combustion engine, a power source shared with other components, etc. However, in other embodiments, the fuel gas separator 130 and the power source 134 driving the fuel gas separator 130 may be configured in any other suitable manner.

As will be explained in more detail below, for the embodiment of FIG. 3, the contactor 128 generally defines a gas inlet 136, a liquid fuel inlet 138, and a fuel/gas mixture outlet 140. Further, the fuel gas separator 130 generally defines a gas outlet 142, a liquid fuel outlet 144, and a fuel/gas mixture inlet 146. The gas inlet 136 of the contactor 128 is in airflow communication with the stripping gas supply path 108, the liquid fuel inlet 138 of the contactor 128 is fluidly connected to the liquid fuel supply path 106, and the fuel/gas mixture outlet 140 of the contactor 128 is fluidly coupled to the inlet 146 of the fuel gas separator 130. Further, the gas outlet 142 of the separator 130 is in airflow communication with the stripping gas return path 112 of the stripping gas flowpath 132, and the liquid fuel outlet 144 of the separator 130 is fluidly connected with the liquid fuel outlet path 110.

Moreover, for the exemplary fuel oxygen reduction unit 104 depicted in FIG. 3, the fuel oxygen reduction unit 104 further includes a first catalyst 148 and a second catalyst 150. For the embodiment shown, the first catalyst 148 and the second catalyst 150 are each arranged in series flow along the stripping gas flowpath 132. More specifically, for the embodiment shown, the first catalyst 148 is positioned within the stripping gas flowpath 132 at a location upstream of the contactor 128 (and downstream of the ullage 118), and the second catalyst 150 is positioned within the stripping gas flowpath 132 at a location downstream of the separator 130 (and upstream of the ullage 118). It will be appreciated, however, that in other embodiments, listed components may be provided in any suitable flow order, not all listed components may be included (see, e.g., FIG. 5), or additional components not listed may be included.

Further, the exemplary fuel oxygen reduction unit 104 depicted in FIG. 3, additionally includes a heat exchanger 152. For the embodiment shown, the heat exchanger 152 is positioned in thermal communication with, and more specifically, in airflow communication with, the stripping gas flowpath 132 at a location downstream of the second catalyst 150, and upstream of the ullage 118. Further, for the embodiment shown the heat exchanger 152 is in thermal communication, or more specifically flow communication, with the liquid fuel supply path 106, such that the liquid fuel supply path 106, or rather, a flow of liquid fuel 116 therethrough, may accept heat from the flow of stripping gas 114 through the stripping gas return path 112 downstream of the second catalyst 150. Such may therefore reduce a temperature of the gas flowing into the ullage 118 of the fuel source 102.

During typical operations, stripping gas 114 is urged through the stripping gas supply flowpath 108 by the ullage pump 124, and across an upstream flame arrester 154. Briefly, the upstream flame arrester 154 may be configured to prevent a flame from crossing into the fuel source 102 from the stripping gas supply path 108, or vice versa. For the embodiment shown, the stripping gas 114 flows from the ullage pump 124 through a pre-heater 125 and into the first catalyst 148. The pre-heater 125 may be an electrical resistance heater, a heat exchanger thermally coupled with another system (not shown), or any other suitable heat source for increasing a temperature of the stripping gas 114. The pre-heater 125 may be configured to increase a temperature of the stripping gas 114 to, or above, an activation temperature of the first catalyst 148 (e.g., a minimum light off temperature of the first catalyst 148 that allows the first catalyst 148 to operate properly). Although the exemplary pre-heater 125 is depicted as a separate component from the first catalyst 148, in other embodiments, the pre-heater 125 may be integrated into the first catalyst 148.

Further, within the first catalyst 148, the oxygen content of the stripping gas 114 is reduced. More specifically, within the first catalyst 148, the potentially relatively oxygen-rich stripping gas 114 may be reacted to reduce the oxygen content thereof. It will be appreciated, however, that first catalyst 148 may be configured in any suitable manner to reduce an oxygen content of the stripping gas 114. For example, in certain embodiments, the first catalyst 148 may be configured to react the fuel-vapor rich stripping gas 114 with elements inside the first catalyst 148 to provide a relatively oxygen-free stripping gas 114 upon exit. For example, the first catalyst 148 may include geometries of catalytic components through which the relatively oxygen-rich stripping gas 114 flows to reduce an oxygen content thereof. Such reaction may utilize at least in part a fuel content of the stripping gas 114 present by virtue of the stripping gas 114 originating in the fuel source 102 (i.e., being in contact with the fuel therein). In one or more of these configurations, a byproduct may be produced, such as water. The water, if produced, may be in vapor form and may continue as part of the stripping gas 114. Alternatively, the water or other byproduct, if produced, may be ducted away from the first catalyst 148 (duct not depicted in the embodiment of FIG. 3). In one or more of these embodiments, the first catalyst 148 (or other gas oxygen reduction unit, discussed below) may be configured to reduce an oxygen content of the stripping gas 114 to less than about three percent (3%) oxygen (O2) by mass, such less than about one percent (1%) oxygen (O2) by mass.

From the first catalyst 148, the stripping gas 114 is provided to the gas inlet 136 of the contactor 128. Simultaneously, liquid fuel 116 is urged by the fuel pump 126 from the fuel source 102 to and through the liquid fuel supply path 106. From the fuel pump 126, the liquid fuel 116 flows through the heat exchanger 152, wherein the liquid fuel 116 may accept heat from the flow of stripping gas 114 through the stripping gas return path 112. From the heat exchanger 152, the liquid fuel 116 flows, still through the liquid fuel supply path 106, to the liquid fuel inlet 138 of the contactor 128. Within the contactor 128, the stripping gas 114 received through the stripping gas inlet 136 is mixed with the flow of liquid fuel 116 received through the liquid fuel inlet 138 to generate a fuel/gas mixture 156. The fuel/gas mixture 156 generated within the contactor 128 is provided to the inlet 146 of the fuel gas separator 130.

Generally, it will be appreciated that during operation of the fuel oxygen reduction unit 104, the liquid fuel 116 provided through the liquid fuel supply path 106 to the contactor 128 may have a relatively high oxygen content. By contrast, the stripping gas 114 provided to the contactor 128 may have a relatively low oxygen content or other specific chemical structure. Within the contactor 128, the liquid fuel 116 is mixed with the stripping gas 114, resulting in the fuel/gas mixture 156. As a result of such mixing a physical exchange may occur whereby at least a portion of the oxygen within the fuel 116 is transferred to the stripping gas 114, such that the fuel component of the mixture 156 has a relatively low oxygen content (as compared to the fuel 116 provided through the liquid fuel supply path 106) and the stripping gas 114 component of the mixture 146 has a relatively high oxygen content (as compared to the stripping gas 114 provided through the stripping gas supply path 108 to the contactor 128).

Referring now also briefly to FIG. 4, providing a close-up, schematic, cross-sectional view of the exemplary fuel gas separator 130 of FIG. 3, it will be appreciated that within the fuel gas separator 130 the stripping gas 114 having a relatively high oxygen content is generally separated from the fuel 116 having a relatively low oxygen content. Specifically, for the embodiment shown, the fuel gas separator 130 defines a central axis 158 and a circumferential direction C extending about the central axis 158. Additionally, the fuel gas separator 130 is configured as a mechanically-driven fuel gas separator 130, or more specifically as a rotary/centrifugal fuel gas separator 130. Accordingly, the fuel gas separator 130 includes an input shaft 160 and a separation assembly 162, the input shaft 160 mechanically coupled to the separation assembly 162, the two components together rotatable about the central axis 158. Further, the input shaft 160 may be mechanically coupled to, and driven by, e.g., a suitable power source 134.

Additionally, the exemplary separation assembly 162 depicted generally includes an inner filter 164 arranged along the central axis 158, and a plurality of paddles 166 positioned radially outward of the inner filter 164. During operation, a rotation of the separation assembly 162 about the central axis 158, and more specifically, a rotation of the plurality of paddles 166 about a central axis 158 (i.e., in the circumferential direction C), may generally force heavier liquid fuel 116 outward and lighter stripping gas 114 inward through the inner filter 164. In such a manner, the liquid fuel 116 may exit through the liquid fuel outlet 144 of the fuel gas separator 130 and the stripping gas 114 may exit through the stripping gas outlet 142 of the fuel gas separator 130, as is indicated.

Accordingly, it will be appreciated that the liquid fuel 116 provided to the liquid fuel outlet 142, having interacted with the stripping gas 114, may have a relatively low oxygen content, such that a relatively high amount of heat may be added thereto with a reduced risk of the fuel coking (i.e., chemically reacting to form solid particles which may clog up or otherwise damage components within the fuel flow path). For example, in at least certain exemplary aspects, the fuel 116 provided to the liquid fuel outlet 144 may an oxygen content of less than about five (5) parts per million (“ppm”), such as less than about three (3) ppm, such as less than about two (2) ppm, such as less than about one (1) ppm, such as less than about 0.5 ppm.

Further, it will be appreciated that the exemplary fuel gas separator 130 depicted in FIG. 4 is provided by way of example only. In other embodiments, the separator 130 may have any other suitable configuration for separating liquid fuel 116 from stripping gas 114. For example, in other embodiments, the separator 130 may instead be configured as another form of centrifugal separator, a gravity-assisted separator, or any other passive separator or powered separator, or combination thereof.

Referring now back to the schematic view of the fuel delivery system 100 in FIG. 3, the liquid fuel 116 from the liquid fuel outlet 144 of the separator 130 is provided through the liquid fuel outlet path 110. The liquid fuel outlet path 110 is fluidly coupled to a liquid fuel source line 168, which may provide the relatively low oxygen content fuel to the one or more engines 56 (see, e.g., FIG. 2). As with the embodiment of FIG. 2, the fuel delivery system 100 further includes a return valve 120 and a return line 122. Depending on a fuel demand of the one or more engines 56 (see FIG. 2), at least a portion of the fuel 116 through the liquid fuel outlet path 110 may be returned to the fuel source 102.

As is also depicted in FIG. 3, it will be appreciated that the exemplary fuel oxygen reduction unit 104 returns the stripping gas 114 utilized to the ullage 118 of the fuel source 102. As noted, the stripping gas 114 to be returned to the ullage 118, i.e., downstream of the separator 130 and through the stripping gas return path 112, may have a relatively high oxygen content. Accordingly, as is also noted above, the fuel oxygen reduction unit 104 further includes the second catalyst 150. The stripping gas 114 from the gas outlet 142 of the separator 130 is provided to the second catalyst 150 wherein an oxygen content of such gas 114 is reduced. The second catalyst 150 is positioned in airflow communication with the stripping gas return path 112 to reduce the oxygen content of stripping gas 114 flowing through the stripping gas return path 112, prior to returning such stripping gas 114 to the ullage 118 of the fuel source 102. It will be appreciated, that in certain exemplary embodiments, the second catalyst 150 may operate in a manner similar to the operations described above with reference to the first catalyst 148. For example, in certain embodiments, the second catalyst 150 may be configured in substantially the same manner as the first catalyst 148, or alternatively, may be configured in any other suitable manner. For example, the second catalyst 150 may react the fuel-vapor rich stripping gas 114 with elements inside the second catalyst 150 to provide a relatively oxygen-free stripping gas 114 upon exit. Inclusion of the second catalyst 150 may ensure the stripping gas 114 returned to the ullage 118 has a desired relatively low oxygen content.

Referring still to the embodiment of FIG. 3, the stripping gas 114 through the stripping gas return path 112 is further cooled downstream of the second catalyst 150 through the heat exchanger 152 (which may be necessary given the presence of the pre-heater 125). For the embodiment shown, the heat exchanger 152 is a gas-to-liquid heat exchanger configured to transfer heat from the stripping gas 114 through the stripping gas return path 112 to a heat exchange fluid, and more specifically, to the liquid fuel 116 flowing through the liquid fuel supply path 106. In such a manner, the heat exchanger 152 may reduce a temperature of the stripping gas 114 through the stripping gas return path 112 and may increase a temperature of the liquid fuel 116 through the liquid fuel supply path 106.

The resulting cooled and relatively low oxygen content stripping gas 114 is then provided through the remainder of the stripping gas flowpath 132, or rather the stripping gas return path 112, back to the ullage 118 of the fuel source 102. In such a manner, the fuel oxygen reduction unit 104 may be configured to simultaneously reduce an oxygen content of a gas within the ullage 118 of the fuel source 102 and reduce an oxygen content of a liquid fuel 116 provided to one or more aircraft engines 52 of a propulsion system 50 of an aircraft 10 including the fuel delivery system 100 (see, e.g., FIGS. 1 and 2). By including a single fuel oxygen reduction system 104 to reduce an oxygen content of fuel to be provided to a plurality of aircraft engines 52, or rather each of the plurality of combustion aircraft engines 52, as well as to reduce an oxygen content of the air within the ullage 118 of the fuel source 102, an overall more efficient fuel delivery system 100 may be provided.

It will be appreciated, however, that in other exemplary embodiments, the fuel delivery system 100 may be configured in any other suitable manner. For example, the fuel delivery system 100 may not provide low oxygen content fuel to each, or even a plurality, of aircraft engines. Further, it will be appreciated that for the embodiment shown, the ullage 118 having the oxygen content of its air being reduced is the ullage 118 of the fuel source 102 providing the liquid fuel 116 through the liquid fuel supply path 106 for the fuel oxygen reduction unit 104. However, in other embodiments, the ullage 118 may be of any other suitable fuel source, such that the fuel oxygen reduction unit 104 is operable with more than one fuel source. For example, in certain embodiments, the fuel oxygen reduction unit 104 may be operable with a primary fuel tank and a separate, secondary fuel tank.

Further, for the embodiment shown, the fuel delivery system 100 further includes a second flame arrester 170 in the stripping gas return path 112 immediately upstream of the ullage 118. The second flame arrester 170 may operate in a similar manner to the first flame arrester 154.

Although not depicted, it will be appreciated that in at least certain exemplary embodiments, the fuel oxygen reduction unit 104 may further include a makeup gas source in airflow communication with, e.g., the stripping gas circulation flowpath 132. For example, the makeup gas source may be in ambient air source, a bleed air source, a stripping gas tank, etc., in airflow communication with the flowpath 132 to provide additional air/stripping gas 114 to the circulation gas flowpath if/as needed.

It will be appreciated, however, that in other exemplary embodiments, the fuel delivery system 100 may instead be configured in any other suitable manner. For example, referring now to FIG. 5, a fuel delivery system 100 in accordance with another exemplary embodiment of the present disclosure is provided. The exemplary fuel delivery system 100 of FIG. 5 may be configured in substantially the same manner as the exemplary fuel delivery system 100 of FIG. 3. For example, the fuel delivery system 100 generally includes a fuel source 102 and fuel oxygen reduction unit 104, with the fuel oxygen reduction unit 104 generally defining a liquid fuel supply path 106 and a liquid fuel outlet path 110, as well as a stripping gas supply path 108 and a stripping gas return path 110. The stripping gas supply path 108 and stripping gas return path 110 are each in airflow communication with the fuel source 102, and more specifically, an ullage 118 of the fuel source 102, and the liquid fuel supply path 106 is in fluid communication with the fuel source 102. Further, the exemplary fuel oxygen reduction unit 104 generally includes a contactor 128 and a separator 130. The contactor 128 is configured to receive a stripping gas 114 flow through a stripping gas inlet 136 in airflow communication with the stripping gas supply path 108 and a liquid fuel 116 flow through a liquid fuel inlet 138 in fluid communication with the liquid fuel supply path 106. The contactor 128 is further configured to mix the stripping gas 114 flow with the liquid fuel 116 flow to form a fuel/gas mixture 156 and to provide such fuel/gas mixture 156 to an inlet 146 of the separator 130. The separator 130 is configured to separate the fuel/gas mixture 156 back into the stripping gas 114 flow, which is provided through the stripping gas return path 112 back to the fuel source 102, and the liquid fuel 116 flow, which is provided through the liquid fuel 116 return path to, e.g., one or more aircraft engines 52.

However, by contrast to the embodiment of FIG. 3, the exemplary fuel delivery system 100 of FIG. 5 may not include both of a first catalyst 148 and a second catalyst 150. Instead, for the exemplary fuel delivery system 100 of FIG. 5, the fuel oxygen reduction unit 104 includes a single catalyst 172 positioned upstream of the contactor 128 and downstream of the ullage 118 of the fuel source 102. In such a manner, the fuel delivery system 100, or rather, the fuel oxygen reduction unit 104 may still reduce an oxygen content of an air within the ullage 118 of the fuel source 102. For example, the catalyst 172 may reduce the oxygen content of the stripping gas 114 flow (received through the stripping gas supply path 108 from the ullage 118) to less than about three percent (3%) oxygen (O2) by mass, such less than about one percent (1%) oxygen (O2) by mass. Although the stripping gas 114 may increase in oxygen content by mixing with the liquid fuel 116 within the contactor 128, prior to being separated back out to the to the stripping gas 114 flow in the separator 130 and returned to the ullage 118 of the fuel tank, the resulting oxygen content of the air within the ullage 118 of the fuel tank may still be less, or substantially less, than it otherwise would be (i.e., an oxygen content of the ullage gas would still be less than an acceptable oxygen level for ullage gas).

Notably, however, as is depicted in phantom, in other embodiments, the single catalyst 172 of the fuel oxygen reduction unit 104 may instead be positioned downstream of the separator 130 and upstream of the ullage 118 of the fuel source 102. With such a configuration, the fuel delivery system 100, or rather, the fuel oxygen reduction unit 104, may still reduce an oxygen content of the air within the ullage 118 of the fuel source 102 and the liquid fuel 116 provided to the one or more aircraft engines 52. For example, with such a configuration, the fuel oxygen reduction unit 104 may maintain an oxygen content of air within the ullage 118 of the fuel source 102 at relatively low level, such that when such air is provided through the stripping gas supply path 108 to the contactor 128 and mixed with liquid fuel 116, it may still be capable of reducing the oxygen content of the fuel to an acceptable or desired level.

Referring still the embodiment of FIG. 5, also by contrast to the embodiment of FIG. 3, the exemplary fuel delivery system 100 includes a heat exchanger 152 downstream of the separator 130 and upstream of the fuel source 102. However, for the embodiment of FIG. 5, the heat exchanger 152 is not in fluid communication with the liquid fuel supply path 106. Instead, the exemplary fuel delivery system 100 includes a separate thermal transfer system 174 and the heat exchanger 152 is thermally coupled to such thermal transfer system 174. The thermal transfer system 174 may be configured to accept heat from the stripping gas 114 through the stripping gas supply path 108 to reduce a temperature of such stripping gas 114. The thermal transfer system 174 may be any suitable thermal transfer system, such as a bypass air system, a lubrication system, etc.

Further, in other embodiments, any other suitable configuration may be provided for the fuel delivery system 100. For example, referring now to FIG. 6, a fuel delivery system 100 in accordance with yet another exemplary embodiment of the present disclosure is provided. The exemplary fuel delivery system 100 of FIG. 6 may be configured in substantially the same manner as the exemplary fuel delivery system 100 of FIG. 3. For example, the fuel delivery system 100 generally includes a fuel source 102 and fuel oxygen reduction unit 104, with the fuel oxygen reduction unit 104 generally defining a liquid fuel supply path 106 and a liquid fuel outlet path 110, as well as a stripping gas supply path 108 and a stripping gas return path 110. The stripping gas supply path 108 and stripping gas return path 110 are each in airflow communication with the fuel source 102, and more specifically, an ullage 118 of the fuel source 102, and further the liquid fuel supply path 106 is in fluid communication with the fuel source 102. Moreover, the exemplary fuel oxygen reduction unit 104 generally includes a contactor 128 and a separator 130.

As with the embodiment of FIG. 3, the fuel delivery system 100 includes a first gas oxygen reduction unit and a second gas oxygen reduction unit. However, by contrast to the embodiment of FIG. 3, for the exemplary fuel delivery system 100 of FIG. 6, the first gas oxygen reduction unit and second gas oxygen reduction unit are not configured as catalysts (i.e., the first catalyst 148 and second catalyst 150), and instead are configured as a first membrane gas oxygen reduction unit 176 and a second membrane gas oxygen reduction unit 178, respectively. The first membrane gas oxygen reduction unit 176 is positioned in the stripping gas supply path 108/stripping gas flowpath 132 downstream (for the embodiment shown) of a boost pump 124 and upstream of a fuel oxygen reduction assembly, or rather upstream of the contactor 128 and separator 130. The second membrane gas oxygen reduction unit 178 is positioned in the stripping gas return path 112/stripping gas flowpath 132 downstream of the fuel oxygen reduction assembly—i.e., the contactor 128 and separator 130 for the embodiment shown—and upstream of the ullage 118 of the fuel source 102.

Each of the first and second membrane gas oxygen reduction units 176, 178 may be configured in any suitable manner for reducing an oxygen content of the stripping gas 114 flowing through the respective portions of the stripping gas flowpath 132. For the embodiments shown, the first and second membrane gas oxygen reduction units 176, 178 each generally define a gas flow chamber 180, through which the stripping gas 114 flows and an oxygen reduction chamber 182, and includes a membrane 184. The oxygen reduction chamber 182 and membrane 184 may have any suitable configuration for extracting oxygen from the stripping gas 114 flowing through the gas flow chamber 180. For example, the oxygen reduction chamber 182 may be a relatively low pressure chamber (e.g., vacuum) and the membrane 184 may be an oxygen permeable membrane allowing for oxygen within the stripping gas 114 within the gas flow chamber 180 to migrate thereacross into the oxygen reduction chamber 182. However, other configurations are contemplated as well, including various other chamber and membrane geometries.

It will be appreciated that with such a configuration, a preheater (such as preheater 125 of FIG. 3) may not be required, such that a heat exchanger (such as heat exchanger 152) may also not be required.

It will further be appreciated that although the exemplary fuel oxygen reduction unit includes the first and second membrane gas oxygen reduction units 176, 178 as the first and second gas oxygen reduction units, in other embodiments the features of the fuel delivery system 100 of FIG. 6 may be combined with one or more of the features of the exemplary fuel delivery systems 100 of FIGS. 3 through 5. For example, in other embodiments one of the first or second membrane gas oxygen reduction units 176, 178 may instead be configured as a catalyst. Alternatively, in other embodiments, the fuel delivery system 100 may only include one of the first or second membrane gas oxygen reduction units 176, 178 (i.e., a single membrane gas oxygen reduction unit for the fuel oxygen reduction unit 104; see, e.g., embodiments described above with reference to FIG. 5).

Moreover, it will be appreciated that in still other exemplary embodiments, other configurations are contemplated as well. For example, referring now to FIG. 7, a fuel delivery system 100 in accordance with still another exemplary embodiment of the present disclosure is provided. The exemplary fuel delivery system 100 of FIG. 7 may be configured in substantially the same manner as the exemplary fuel delivery system 100 of FIG. 3. For example, the fuel delivery system 100 generally includes a fuel source 102 and fuel oxygen reduction unit 104, with the fuel oxygen reduction unit 104 generally defining a liquid fuel supply path 106 and a liquid fuel outlet path 110, as well as a stripping gas supply path 108 and a stripping gas return path 110. Further, the fuel oxygen reduction unit 104 includes a first gas oxygen reduction unit 186 and a second gas oxygen reduction unit 188 positioned within the stripping gas supply path 108 and return path 110, respectively, upstream and downstream, respectively, of a fuel oxygen reduction assembly.

However, for the embodiment depicted, the fuel oxygen reduction assembly is not configured as a contactor and a separator, and instead is configured as a membrane fuel oxygen reduction unit 190. The membrane fuel oxygen reduction unit 190 defines a stripping gas chamber 192 defining a gas inlet 194 and a gas outlet 196, as well as a fuel chamber 198, defining a fuel inlet 200 and a fuel outlet 202. The membrane fuel oxygen reduction unit 190 further includes a membrane 204 positioned between the stripping gas chamber 192 and fuel chamber 198. The membrane 204 may be any suitable membrane for allowing the migration of oxygen within the liquid fuel 116 flowing through the fuel chamber 198 to the stripping gas 114 flowing through the stripping gas chamber 192. For example, the membrane 204 may be any suitable oxygen permeable membrane.

Such a configuration may allow for the simultaneous reduction of oxygen within the ullage 118 and the liquid fuel 116 provided to the engines 50, while utilizing less mechanical work.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A fuel system for an aircraft, the fuel system comprising: a fuel source; and a fuel oxygen reduction unit defining a liquid fuel supply path, a stripping gas supply path, a liquid fuel outlet path, and a stripping gas return path, wherein the stripping gas return path and the stripping gas supply path are each in airflow communication with the fuel source, and wherein the fuel oxygen reduction unit comprises: a fuel oxygen reduction assembly comprising a contactor configured for mixing a liquid fuel supplied from the fuel source and a stripping gas supplied from the fuel source, a first gas oxygen reduction unit positioned in the stripping gas supply path upstream of the contactor and downstream of the fuel source, and a second gas oxygen reduction unit positioned in the stripping gas return path upstream of the fuel source, downstream of the contactor, and downstream of the first gas oxygen reduction unit such that the contactor is disposed between the first gas oxygen reduction unit and the second gas oxygen reduction unit and the fuel source is disposed downstream of the second gas oxygen reduction unit such that the fuel source is disposed between the first gas oxygen reduction unit and the second gas oxygen reduction unit.
 2. The fuel system of claim 1, wherein the fuel source comprises a fuel tank, and wherein the stripping gas supply path and the stripping gas return path are each in airflow communication with an ullage of the fuel tank.
 3. The fuel system of claim 2, wherein the liquid fuel supply path is fluidly connected to the fuel source, and wherein the liquid fuel outlet path is in fluid communication with at least one engine.
 4. The fuel system of claim 1, wherein the fuel oxygen reduction unit is configured to receive a flow of gas through the stripping gas supply path and a flow of liquid fuel through the liquid fuel supply path, and wherein the fuel oxygen reduction unit is configured to reduce an oxygen content of the flow of gas and the flow of liquid fuel.
 5. The fuel system of claim 1, wherein the fuel oxygen reduction assembly is in fluid communication with the liquid fuel supply path and a liquid fuel return path, the liquid fuel return path being in fluid communication with the fuel source, and further in airflow communication with the stripping gas supply path and stripping gas return path.
 6. The fuel system of claim 5, wherein the fuel oxygen reduction assembly comprises: the contactor fluidly connected to the liquid fuel supply path and the stripping gas supply path for mixing the liquid fuel from a liquid fuel flow from the liquid fuel supply path with the stripping gas from a stripping gas flow from the stripping gas supply path to form a fuel/gas mixture; and a separator fluidly connected to the contactor for receiving the fuel/gas mixture and separating the fuel/gas mixture back into the stripping gas flow and the liquid fuel flow, the separator fluidly connected to the liquid fuel outlet path and the stripping gas return path for providing the liquid fuel flow to the liquid fuel outlet path and the stripping gas flow to the stripping gas return path.
 7. The fuel system of claim 1, wherein the fuel oxygen reduction assembly comprises: a separator fluidly connected to the contactor for receiving the liquid fuel and stripping gas mixture and separating the liquid fuel and stripping gas mixture back into the stripping gas flow and the liquid fuel flow.
 8. The fuel system of claim 1, wherein the fuel oxygen reduction assembly comprises: a separator fluidly connected to the liquid fuel outlet path and the stripping gas return path for separating the liquid fuel from the stripping gas to provide the liquid fuel to the liquid fuel flow of the liquid fuel outlet path and the stripping gas to the stripping gas flow of the stripping gas return path.
 9. The fuel system of claim 1, wherein at least one of the first gas oxygen reduction unit and the second gas oxygen reduction unit is a catalyst.
 10. The fuel system of claim 9, wherein the fuel oxygen reduction unit further comprises: a pre-heater thermally coupled to the stripping gas supply path; and a heat exchanger thermally coupled to the stripping gas return path.
 11. The fuel system of claim 1, wherein the fuel oxygen reduction unit further comprises: an ullage pump in airflow communication with the stripping gas supply path.
 12. A propulsion system for an aircraft comprising an aircraft engine; and a fuel system comprising a fuel source; and a fuel oxygen reduction unit defining a liquid fuel supply path, a stripping gas supply path, a liquid fuel outlet path, and a stripping gas return path, wherein the stripping gas return path and stripping gas supply path are each in airflow communication with the fuel source, wherein the fuel oxygen reduction unit comprises: a fuel oxygen reduction assembly comprising a contactor configured for mixing a liquid fuel supplied from the fuel source and a stripping gas supplied from the fuel source, a first gas oxygen reduction unit positioned in the stripping gas supply path upstream of the contactor and downstream of the fuel source, and a second gas oxygen reduction unit positioned in the stripping gas return path upstream of the fuel source, and downstream of the contactor, and downstream of the first gas oxygen reduction unit such that the contactor is disposed between the first gas oxygen reduction unit and the second gas oxygen reduction unit and the fuel source is disposed downstream of the second gas oxygen reduction unit such that the fuel source is disposed between the first gas oxygen reduction unit and the second gas oxygen reduction unit, and wherein the liquid fuel outlet path is in fluid communication with the aircraft engine for providing liquid fuel to the aircraft engine.
 13. The propulsion system of claim 12, wherein the aircraft engine is a first aircraft engine, and wherein the propulsion system further comprises: a second aircraft engine, wherein the liquid fuel outlet path is in fluid communication with both the first aircraft engine and the second aircraft engine for providing liquid fuel to both the first aircraft engine and the second aircraft engine.
 14. The propulsion system of claim 12, wherein the fuel oxygen reduction assembly is in fluid communication with the liquid fuel supply path and a liquid fuel return path, the liquid fuel return path being in fluid communication with the fuel source, and further in airflow communication with the stripping gas supply path and stripping gas return path.
 15. The propulsion system of claim 14, wherein the fuel oxygen reduction assembly comprises: the contactor fluidly connected to the liquid fuel supply path and the stripping gas supply path for mixing the liquid fuel from a liquid fuel flow from the liquid fuel supply path with the stripping gas from a stripping gas flow from the stripping gas supply path to form a fuel/gas mixture; and a separator fluidly connected to the contactor for receiving the fuel/gas mixture and separating the fuel/gas mixture back into the stripping gas flow and the liquid fuel flow, the separator fluidly connected to the liquid fuel outlet path and the stripping gas return path for providing the liquid fuel flow to the liquid fuel outlet path and the stripping gas flow to the stripping gas return path.
 16. The propulsion system of claim 12, wherein the fuel oxygen reduction assembly comprises: a separator fluidly connected to the contactor for receiving the liquid fuel and stripping gas mixture and separating the liquid fuel and stripping gas mixture back into the stripping gas flow and the liquid fuel flow and the stripping gas flow.
 17. The propulsion system of claim 14, wherein each of the first gas oxygen reduction unit and the second gas oxygen reduction unit is a membrane gas oxygen reduction unit or a catalyst.
 18. The propulsion system of claim 12, wherein the liquid fuel supply path is fluidly connected to the fuel source for receiving a liquid fuel flow from the fuel source. 